CHAPTER 18 Regulation of the Heart and Vasculature and large bowel. Skeletal muscle and skin do not receive parasympathetic innervation. The effect of cholinergic fibers on total vascular resistance is small because only a small proportion of the resistance vessels of the body receive parasympathetic fibers.
Inhemorrhagicshock,theresistancevesselsconstrictandtherebyassistinthemaintenanceofnormalarterialbloodpressure.Witharterialhypotension,theenhancedarteriolarconstrictionalsoleadstoasmallmobilizationofbloodfromthetissuebyvirtueofrecoilofthepostarteriolarvesselswhenintraluminalpressureisreduced.Furthermore,extravascularfluidismobilizedbecauseofgreaterfluidabsorptionintothecapillariesinresponsetotheloweredcapillaryhydrostaticpressure.
Stimulation of the parasympathetic fibers to the salivary glands induces marked vasodilation. A vasodilator polypeptide, bradykinin, formed locally from the action of an enzyme on a plasma protein substrate in the glandular lymphatic vessels, mediates this vasodilation. Bradykinin is formed in other exocrine glands, such as the lacrimal and sweat glands. Its presence in sweat may be partly responsible for the dilation of cutaneous blood vessels.
Epinephrine and norepinephrine exert a powerful effect on peripheral blood vessels. In skeletal muscle, low concentrations of epinephrine dilate resistance vessels (β2-adrenergic effect), but high concentrations produce constriction (α1adrenergic effect), as noted in
Table 14.1.
In all vascular beds, the primary effect of norepinephrine is vasoconstriction. When stimulated, the adrenal gland can release epinephrine and norepinephrine into the systemic circulation. However, under physiological conditions, the effect of catecholamine release from the adrenal medulla is less important than norepinephrine release from sympathetic nerve endings.
Table 14.1 ). Some are released from the endothelium (e.g., nitric oxide, endothelin, thromboxane A2), whereas others are derived from perivascular tissues (e.g., histamine, adenosine, angiotensin II).
Areas of the cerebral medulla that mediate sympathetic and vagal effects are under the influence of neural impulses that originate in the baroreceptors, chemoreceptors, hypothalamus, cerebral cortex, and skin. These areas of the medulla are also affected by changes in the blood concentrations of CO2 and O2.
The baroreceptors (or pressoreceptors) are stretch receptors located in the carotid sinuses and in the aortic arch (
Figs. 18.25 18.26 ). The carotid sinuses are the slightly
Fig. 18.25 Diagrammaticrepresentationofthecarotidsinusandcarotidbodyandtheirinnervation.(RedrawnfromAdamsWE.The Comparative Morphology of the Carotid Body and Carotid Sinus.
Fig. 18.26 Anteriorviewoftheaorticarchshowingtheinnervationoftheaorticbodiesandbaroreceptors.(ModifiedfromNonidezJF.Anat Rec. 1937;69:299.)